This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:
3GPP third generation partnership project
ARQ automatic repeat request
BSR buffer status report
CA carrier aggregation
CC component carrier
CE control element
DL downlink (eNB to UE)
eNB EUTRAN Node B (evolved Node B/base station)
E-UTRAN evolved UTRAN (LTE)
HARQ hybrid ARQ
LTE long term evolution
MAC medium access control
NACK negative acknowledgment
PCC primary component carrier
PDCCH physical downlink control channel
PUCCH physical uplink control channel
RACH random access channel
RRC radio resource control
SCC secondary component carrier
SR scheduling request
SRS sounding reference signal
UE user equipment
UL uplink (UE to eNB)
UTRAN universal terrestrial radio access network
In the communication system known as evolved UTRAN (E-UTRAN, also referred to as UTRAN-LTE or E-UTRA), the LTE Release 8 is completed, the LTE Release 9 is being standardized, and the LTE Release 10 is currently under development within the 3GPP. In LTE the downlink access technique is orthogonal frequency multiple division access OFDMA, and the uplink access technique is single carrier, frequency division multiple access SC-FDMA. These access techniques are expected to continue in LTE Release 10.
FIG. 1 reproduces FIG. 4.1 of 3GPP TS 36.300, V8.6.0 (2008-09), and shows the overall architecture of the E-UTRAN system. The EUTRAN system includes eNBs, providing the EUTRA user plane and control plane (RRC) protocol terminations towards the UE. The eNBs are interconnected with each other by means of an X2 interface. The eNBs are also connected by means of an S1 interface to an evolved packet core, more specifically to a MME and to a Serving Gateway. The S1 interface supports a many to many relationship between MMEs/Serving Gateways and the eNBs.
Of particular interest herein are the further releases of 3GPP LTE targeted towards future international mobile telecommunications (IMT)-advanced systems, referred to herein for convenience simply as LTE-Advanced (LTE-A). LTE-A is directed toward extending and optimizing the 3GPP LTE Release 8 radio access technologies to provide higher data rates at very low cost. LTE-A will most likely be part of LTE Release 10 which is to be backward compatible with LTE Release 8 and to include bandwidth extensions beyond 20 MHz, among others. For an overview see for example 3GPP TR 36.913 v9.0.0 (2009-12) Requirements for further advancements for Evolved Universal Terrestrial Radio Access (E-UTRA) (LTE_Advanced) (Release 9).
The bandwidth extension in LTE Release 10 is to be done via carrier aggregation (CA), in which several Release 8 compatible carriers are aggregated together to form a system bandwidth. This is shown by example at FIG. 2 in which there are five Release 8 compatible CCs aggregated to form one larger LTE Release 10 bandwidth. Existing Release 8 terminals can receive and/or transmit on one of the CCs for backward compatibility, while future LTE-A terminals could potentially receive/transmit on multiple CCs at the same time to give the eNB greater scheduling flexibility while increasing data throughput.
In cellular radio systems generally, the user equipments adjust the timing of their transmissions to the base station so that the times when each transmitted symbol arrives at the eNB's receiver are within at most a small timing offset of the times when the eNB is expecting them. This example is uplink time alignment.
Specifically for the LTE Release 8/9 system, the UE synchronizes its uplink transmissions by first synchronizing with the eNB's transmissions in the downlink (by detecting bit and frame timing), and then transmitting at a fixed delay (stipulated by a the controlling radio standard) relative to the downlink, the delay further reduced by a timing advance. The timing advance compensates for the round trip propagation delay between the eNB and the UE and varies with time, due to the UEs mobility.
The UE operating in an LTE Release 8/9 system obtains the correct timing during its initial entry into the network, which is done via a RACH procedure that does not require pre-existing timing synchronization between the accessing UE and the network. Once established in the network the UE's timing is kept in alignment by a MAC CE, which the eNB transmits to a specific UE when it detects that the UE's uplink transmission timing is in error.
The UE tracks its timing alignment by means of a time alignment timer, which is started or restarted whenever a timing correction is received (either in a MAC CE or a Random Access Response). If the timing alignment expires, the UE is required to act as if timing alignment has been lost. For the LTE Release 8 cellular radio system, a UE whose timing alignment timer has expired is required [by 3GPP TS36.321 section 5.2] to:                flush all HARQ buffers;        notify RRC to release PUCCH/SRS; and        clear any configured downlink assignments and uplink grants.        
In LTE Release 8 the UE does not initiate RACH transmissions to keep in uplink time alignment, these must be triggered by the eNB. The eNB can keep a non-transmitting UE in uplink time alignment by instructing it, via a PDCCH order that is transmitted on the PDCCH to transmit on the RACH.
Flushing the HARQ buffers deletes all information pending transmission/re-transmission from the HARQ level. Releasing the PUCCH/SRS removes the uplink control channel that the UE uses to transmit ACK/NACK reports, CQI measurement reports and SR bits that are used to indicate that the UE has uplink data to send. Clearing the configured downlink assignments and uplink grants removes all current or pending opportunities/requirements for uplink transmission.
Once a UE loses its uplink time alignment in an LTE Release 8 system it is not permitted to transmit in the uplink, except on the RACH. In the current 3GPP LTE specification, expiry of the time alignment timer itself does not trigger the UE to initiate a RACH transmission.
The UE with lost timing synchronization continues to receive the PDCCH control channel in LTE Release 8, but does not transmit in the uplink, unless one of the following occurs:                It receives on its downlink control channel a PDCCH order command that triggers it to transmit on RACH and receive a time alignment correction in response. This will bring the UE into uplink time alignment and enable it to transmit in the uplink. The Time Alignment Timer is restarted.        It has new uplink data to transmit which triggers the sending of buffer status report (BSR). This will, in turn, trigger the UE to transmit on RACH in order receive a grant to send the BSR. The Random Access Response will also provide the timing correction that will bring the UE into uplink time alignment.        
But in LTE Release 8/9 there is no CA, and so each UE tracks its timing synchronization with the eNB by a single timing alignment timer. In the CA arrangement of spectrum as in LTE Release 10, a single UE may be assigned radio resources on more than one CC. In some cases more than one CC is aligned in time and so the same timing alignment timer can be used for them all. Such timing-dependent CCs for which the UE tracks timing synchronization by a single timing alignment timer are termed a timing advance group of CCs, and there may be only one or more than one CC in any timing advance group. In other cases at least two of the CCs assigned to the LTE Release 10 UE are timing independent, and so the UE must maintain a separate timing advance timer for each of the different timing advance groups it is assigned. Currently it is not specified how a UE should respond in the case where one but not all timing alignment timer expires for a UE that is operating with two or more timing advance groups.
To extend the LTE Release 8/9 rules for the case of lost timing synchronization on less than all configured timing advance groups would be to dis-allow the UE from transmitting in the uplink on any of its configured CCs. This result would waste the capacity of the uplink carriers for which the UE still has timing alignment. Alternatively, the eNB can keep the UE in timing alignment for all its configured carriers by way of a MAC CE and/or PDCCH order procedures. However, errors in acknowledgement (NACK to ACK errors in which a UE's NACK sent with improper timing is interpreted by the eNB as an ACK) at the HARQ level for such MAC CE corrections can mean that the eNB thinks that the UE is time aligned whereas one or more of that same UE's time alignment timers have expired. In principle, the eNB could also allow timing alignment to temporarily lapse for one or more of the UE's configured timing groups, but not the timing group which includes the UE's primary CC. But in this case the UE will not know whether or not the expired timing alignment timer was intended by the network.